They could trade the pick anytime they want to, however the trades most likely won't happen until Draft day. Reason being that the team that picks ahead of you will have a big affect on what trade offers you might get. For example, if the Tampa rumors are true, they would trade up to the #2 spot only if San Francisco does not draft Alex Smith. Trading for the #2 pick now makes no sense, because the person you are trading up for is not 100% guaranteed to be there.
Yeah you won't see any deal happening until we're on the clock most likely. Unless San Francisco announces that they've agreed to a contract with Aaron Rodgers during the week then you may see the pick dealt before Saturday.